Chroma coding enhancement in cross-component sample adaptive offset

ABSTRACT

An electronic apparatus performs a method of decoding a video signal. The method comprises: receiving the video signal that includes a first component and a second component in a first picture frame; receiving, from the video signal, a plurality of sample offsets associated with the second component in the first picture frame; deriving a first class index for the second component from a first set of one or more samples of the first component relative to each sample of the second component; selecting a first sample offset from the plurality of sample offsets for the second component according to the first class index; and obtaining a cross-component offsetted sample value of the second component based on the first sample offset in the first picture frame. In some embodiments, the first picture frame is divided into a plurality of regions, and a different classifier is used for each of the plurality of regions.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a continuation application of InternationalApplication No. PCT/US2021/044822, entitled “CHROMA CODING ENHANCEMENTIN CROSS-COMPONENT SAMPLE ADAPTIVE OFFSET” filed on Aug. 5, 2021, whichis based upon and claims priority to U.S. Provisional Patent ApplicationNo. 63/065,351, entitled “Cross-component Sample Adaptive Offset” filedon Aug. 13, 2020, each of which is incorporated by reference in itsentirety.

TECHNICAL FIELD

The present application generally relates to video coding andcompression, and more specifically, to methods and apparatus onimproving the chroma coding efficiency.

BACKGROUND

Digital video is supported by a variety of electronic devices, such asdigital televisions, laptop or desktop computers, tablet computers,digital cameras, digital recording devices, digital media players, videogaming consoles, smart phones, video teleconferencing devices, videostreaming devices, etc. The electronic devices transmit, receive,encode, decode, and/or store digital video data by implementing videocompression/decompression standards. Some well-known video codingstandards include Versatile Video Coding (VVC), High Efficiency VideoCoding (HEVC, also known as H.265 or MPEG-H Part 2) and Advanced VideoCoding (AVC, also known as H.264 or MPEG-4 Part 10), which are jointlydeveloped by ISO/IEC MPEG and ITU-T VCEG. AOMedia Video 1 (AV1) wasdeveloped by Alliance for Open Media (AOM) as a successor to itspreceding standard VP9. Audio Video Coding (AVS), which refers todigital audio and digital video compression standard, is another videocompression standard series developed by the Audio and Video CodingStandard Workgroup of China.

Video compression typically includes performing spatial (intra frame)prediction and/or temporal (inter frame) prediction to reduce or removeredundancy inherent in the video data. For block-based video coding, avideo frame is partitioned into one or more slices, each slice havingmultiple video blocks, which may also be referred to as coding treeunits (CTUs). Each CTU may contain one coding unit (CU) or recursivelysplit into smaller CUs until the predefined minimum CU size is reached.Each CU (also named leaf CU) contains one or multiple transform units(TUs) and each CU also contains one or multiple prediction units (PUs).Each CU can be coded in either intra, inter or IBC modes. Video blocksin an intra coded (I) slice of a video frame are encoded using spatialprediction with respect to reference samples in neighboring blockswithin the same video frame. Video blocks in an inter coded (P or B)slice of a video frame may use spatial prediction with respect toreference samples in neighboring blocks within the same video frame ortemporal prediction with respect to reference samples in other previousand/or future reference video frames.

Spatial or temporal prediction based on a reference block that has beenpreviously encoded, e.g., a neighboring block, results in a predictiveblock for a current video block to be coded. The process of finding thereference block may be accomplished by block matching algorithm.Residual data representing pixel differences between the current blockto be coded and the predictive block is referred to as a residual blockor prediction errors. An inter-coded block is encoded according to amotion vector that points to a reference block in a reference frameforming the predictive block, and the residual block. The process ofdetermining the motion vector is typically referred to as motionestimation. An intra coded block is encoded according to an intraprediction mode and the residual block. For further compression, theresidual block is transformed from the pixel domain to a transformdomain, e.g., frequency domain, resulting in residual transformcoefficients, which may then be quantized. The quantized transformcoefficients, initially arranged in a two-dimensional array, may bescanned to produce a one-dimensional vector of transform coefficients,and then entropy encoded into a video bitstream to achieve even morecompression.

The encoded video bitstream is then saved in a computer-readable storagemedium (e.g., flash memory) to be accessed by another electronic devicewith digital video capability or directly transmitted to the electronicdevice wired or wirelessly. The electronic device then performs videodecompression (which is an opposite process to the video compressiondescribed above) by, e.g., parsing the encoded video bitstream to obtainsyntax elements from the bitstream and reconstructing the digital videodata to its original format from the encoded video bitstream based atleast in part on the syntax elements obtained from the bitstream, andrenders the reconstructed digital video data on a display of theelectronic device.

With digital video quality going from high definition, to 4K×2K or even8K×4K, the amount of vide data to be encoded/decoded growsexponentially. It is a constant challenge in terms of how the video datacan be encoded/decoded more efficiently while maintaining the imagequality of the decoded video data.

SUMMARY

The present application describes implementations related to video dataencoding and decoding and, more particularly, to methods and apparatuson improving the coding efficiency of chroma coding, including improvingthe coding efficiency by exploring cross-component relationship betweenluma and chroma components.

According to a first aspect of the present application, a method ofdecoding a video signal comprises: receiving the video signal thatincludes a first component and a second component in a first pictureframe; receiving, from the video signal, a plurality of sample offsetsassociated with the second component in the first picture frame;deriving a first class index for the second component from a first setof one or more samples of the first component relative to each sample ofthe second component; selecting a first sample offset from the pluralityof sample offsets for the second component according to the first classindex; and obtaining a cross-component offsetted sample value of thesecond component based on the first sample offset in the first pictureframe.

In some embodiments, the method of decoding video signal furthercomprises: deriving a second class index for the second component in asecond region in the first picture frame from a second set of one ormore samples of the first component relative to each sample of thesecond component; selecting a second sample offset from the plurality ofsample offsets for the second component according to the second classindex; and obtaining a cross-component offsetted sample value of thesecond component based on the second sample offset in the second regionin the first picture frame; wherein deriving the first class index forthe second component in the first picture frame is in a first region inthe first picture frame.

In some embodiments, the first picture frame is divided into a pluralityof regions, and a different class index is used for each of theplurality of regions.

According to a second aspect of the present application, an electronicapparatus includes one or more processing units, memory and a pluralityof programs stored in the memory. The programs, when executed by the oneor more processing units, cause the electronic apparatus to perform themethod of coding video data as described above.

According to a third aspect of the present application, a non-transitorycomputer readable storage medium stores a plurality of programs forexecution by an electronic apparatus having one or more processingunits. The programs, when executed by the one or more processing units,cause the electronic apparatus to perform the method of coding videodata as described above.

BRIEF DESCRIPTION OF DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the implementations and are incorporated herein andconstitute a part of the specification, illustrate the describedimplementations and together with the description serve to explain theunderlying principles. Like reference numerals refer to correspondingparts.

FIG. 1 is a block diagram illustrating an exemplary video encoding anddecoding system in accordance with some implementations of the presentdisclosure.

FIG. 2 is a block diagram illustrating an exemplary video encoder inaccordance with some implementations of the present disclosure.

FIG. 3 is a block diagram illustrating an exemplary video decoder inaccordance with some implementations of the present disclosure.

FIGS. 4A through 4E are block diagrams illustrating how a frame isrecursively partitioned into multiple video blocks of different sizesand shapes in accordance with some implementations of the presentdisclosure.

FIG. 5 is a block diagram depicting the four gradient patterns used inSample Adaptive Offset (SAO) in accordance with some implementations ofthe present disclosure.

FIG. 6A is a block diagram illustrating the system and process ofCross-Component Sample Adaptive Offset (CCSAO) according to someimplementations of the present disclosure.

FIG. 6B is a block diagram illustrating the system and process of CCSAOapplied in parallel with ESAO in the AVS standard according to someimplementations of the present disclosure.

FIG. 7 is a block diagram illustrating a sample process using CCSAO inaccordance with some implementations of the present disclosure.

FIG. 8 is a block diagram illustrating that CCSAO process is interleavedto vertical and horizontal deblocking filter (DBF) in accordance withsome implementations of the present disclosure.

FIG. 9 is a flowchart illustrating an exemplary process of decodingvideo signal using cross-component correlation in accordance with someimplementations of the present disclosure.

FIG. 10 is a block diagram showing a class index using different lumasample position for classification in accordance with someimplementations of the present disclosure.

FIG. 11 is a block diagram of a sample process illustrating that besidesluma, the other cross-component collocated and neighboring chromasamples are also fed into CCSAO classification in accordance with someimplementations of the present disclosure.

FIG. 12 illustrates exemplary class indexes by replacing the collocatedluma sample value with a value obtained by weighing collocated andneighboring luma samples in accordance with some implementations of thepresent disclosure.

FIG. 13A is a block diagram illustrating CCSAO is not applied on thecurrent chroma sample if any of the collocated and neighboring lumasamples used for classification is outside the current picture inaccordance with some implementations of the present disclosure.

FIG. 13B is a block diagram illustrating CCSAO is applied on the currentchroma sample if any of the collocated and neighboring luma samples usedfor classification is outside the current picture in accordance withsome implementations of the present disclosure.

FIG. 14 is a flowchart illustrating an exemplary process of decodingvideo signal using cross-component correlation in a picture frame inaccordance with some implementations of the present disclosure.

DETAILED DESCRIPTION

Reference will now be made in detail to specific implementations,examples of which are illustrated in the accompanying drawings. In thefollowing detailed description, numerous non-limiting specific detailsare set forth in order to assist in understanding the subject matterpresented herein. But it will be apparent to one of ordinary skill inthe art that various alternatives may be used without departing from thescope of claims and the subject matter may be practiced without thesespecific details. For example, it will be apparent to one of ordinaryskill in the art that the subject matter presented herein can beimplemented on many types of electronic devices with digital videocapabilities.

The first generation AVS standard includes Chinese national standard“Information Technology, Advanced Audio Video Coding, Part 2: Video”(known as AVS1) and “Information Technology, Advanced Audio Video CodingPart 16: Radio Television Video” (known as AVS+). It can offer around50% bit-rate saving at the same perceptual quality compared to MPEG-2standard. The second generation AVS standard includes the series ofChinese national standard “Information Technology, Efficient MultimediaCoding” (knows as AVS2), which is mainly targeted at the transmission ofextra HD TV programs. The coding efficiency of the AVS2 is double ofthat of the AVS+. Meanwhile, the AVS2 standard video part was submittedby Institute of Electrical and Electronics Engineers (IEEE) as oneinternational standard for applications. The AVS3 standard is one newgeneration video coding standard for UHD video application aiming atsurpassing the coding efficiency of the latest international standardHEVC, which provides approximately 30% bit-rate savings over the HEVCstandard. In March 2019, at the 68-th AVS meeting, the AVS3-P2 baselinewas finished, which provides approximately 30% bit-rate savings over theHEVC standard. Currently, one reference software, called highperformance model (HPM), is maintained by the AVS group to demonstrate areference implementation of the AVS3 standard. Like the HEVC, the AVS3standard is built upon the block-based hybrid video coding framework.

FIG. 1 is a block diagram illustrating an exemplary system 10 forencoding and decoding video blocks in parallel in accordance with someimplementations of the present disclosure. As shown in FIG. 1, system 10includes a source device 12 that generates and encodes video data to bedecoded at a later time by a destination device 14. Source device 12 anddestination device 14 may comprise any of a wide variety of electronicdevices, including desktop or laptop computers, tablet computers, smartphones, set-top boxes, digital televisions, cameras, display devices,digital media players, video gaming consoles, video streaming device, orthe like. In some implementations, source device 12 and destinationdevice 14 are equipped with wireless communication capabilities.

In some implementations, destination device 14 may receive the encodedvideo data to be decoded via a link 16. Link 16 may comprise any type ofcommunication medium or device capable of moving the encoded video datafrom source device 12 to destination device 14. In one example, link 16may comprise a communication medium to enable source device 12 totransmit the encoded video data directly to destination device 14 inreal-time. The encoded video data may be modulated according to acommunication standard, such as a wireless communication protocol, andtransmitted to destination device 14. The communication medium maycomprise any wireless or wired communication medium, such as a radiofrequency (RF) spectrum or one or more physical transmission lines. Thecommunication medium may form part of a packet-based network, such as alocal area network, a wide-area network, or a global network such as theInternet. The communication medium may include routers, switches, basestations, or any other equipment that may be useful to facilitatecommunication from source device 12 to destination device 14.

In some other implementations, the encoded video data may be transmittedfrom output interface 22 to a storage device 32. Subsequently, theencoded video data in storage device 32 may be accessed by destinationdevice 14 via input interface 28. Storage device 32 may include any of avariety of distributed or locally accessed data storage media such as ahard drive, Blu-ray discs, DVDs, CD-ROMs, flash memory, volatile ornon-volatile memory, or any other suitable digital storage media forstoring encoded video data. In a further example, storage device 32 maycorrespond to a file server or another intermediate storage device thatmay hold the encoded video data generated by source device 12.Destination device 14 may access the stored video data from storagedevice 32 via streaming or downloading. The file server may be any typeof computer capable of storing encoded video data and transmitting theencoded video data to destination device 14. Exemplary file serversinclude a web server (e.g., for a website), an FTP server, networkattached storage (NAS) devices, or a local disk drive. Destinationdevice 14 may access the encoded video data through any standard dataconnection, including a wireless channel (e.g., a Wi-Fi connection), awired connection (e.g., DSL, cable modem, etc.), or a combination ofboth that is suitable for accessing encoded video data stored on a fileserver. The transmission of encoded video data from storage device 32may be a streaming transmission, a download transmission, or acombination of both.

As shown in FIG. 1, source device 12 includes a video source 18, a videoencoder 20 and an output interface 22. Video source 18 may include asource such as a video capture device, e.g., a video camera, a videoarchive containing previously captured video, a video feed interface toreceive video from a video content provider, and/or a computer graphicssystem for generating computer graphics data as the source video, or acombination of such sources. As one example, if video source 18 is avideo camera of a security surveillance system, source device 12 anddestination device 14 may form camera phones or video phones. However,the implementations described in the present application may beapplicable to video coding in general, and may be applied to wirelessand/or wired applications.

The captured, pre-captured, or computer-generated video may be encodedby video encoder 20. The encoded video data may be transmitted directlyto destination device 14 via output interface 22 of source device 12.The encoded video data may also (or alternatively) be stored ontostorage device 32 for later access by destination device 14 or otherdevices, for decoding and/or playback. Output interface 22 may furtherinclude a modem and/or a transmitter.

Destination device 14 includes an input interface 28, a video decoder30, and a display device 34. Input interface 28 may include a receiverand/or a modem and receive the encoded video data over link 16. Theencoded video data communicated over link 16, or provided on storagedevice 32, may include a variety of syntax elements generated by videoencoder 20 for use by video decoder 30 in decoding the video data. Suchsyntax elements may be included within the encoded video datatransmitted on a communication medium, stored on a storage medium, orstored a file server.

In some implementations, destination device 14 may include a displaydevice 34, which can be an integrated display device and an externaldisplay device that is configured to communicate with destination device14. Display device 34 displays the decoded video data to a user, and maycomprise any of a variety of display devices such as a liquid crystaldisplay (LCD), a plasma display, an organic light emitting diode (OLED)display, or another type of display device.

Video encoder 20 and video decoder 30 may operate according toproprietary or industry standards, such as VVC, HEVC, MPEG-4, Part 10,Advanced Video Coding (AVC), AVS, or extensions of such standards. Itshould be understood that the present application is not limited to aspecific video coding/decoding standard and may be applicable to othervideo coding/decoding standards. It is generally contemplated that videoencoder 20 of source device 12 may be configured to encode video dataaccording to any of these current or future standards. Similarly, it isalso generally contemplated that video decoder 30 of destination device14 may be configured to decode video data according to any of thesecurrent or future standards.

Video encoder 20 and video decoder 30 each may be implemented as any ofa variety of suitable encoder circuitry, such as one or moremicroprocessors, digital signal processors (DSPs), application specificintegrated circuits (ASICs), field programmable gate arrays (FPGAs),discrete logic, software, hardware, firmware or any combinationsthereof. When implemented partially in software, an electronic devicemay store instructions for the software in a suitable, non-transitorycomputer-readable medium and execute the instructions in hardware usingone or more processors to perform the video coding/decoding operationsdisclosed in the present disclosure. Each of video encoder 20 and videodecoder 30 may be included in one or more encoders or decoders, eitherof which may be integrated as part of a combined encoder/decoder (CODEC)in a respective device.

FIG. 2 is a block diagram illustrating an exemplary video encoder 20 inaccordance with some implementations described in the presentapplication. Video encoder 20 may perform intra and inter predictivecoding of video blocks within video frames. Intra predictive codingrelies on spatial prediction to reduce or remove spatial redundancy invideo data within a given video frame or picture. Inter predictivecoding relies on temporal prediction to reduce or remove temporalredundancy in video data within adjacent video frames or pictures of avideo sequence.

As shown in FIG. 2, video encoder 20 includes video data memory 40,prediction processing unit 41, decoded picture buffer (DPB) 64, summer50, transform processing unit 52, quantization unit 54, and entropyencoding unit 56. Prediction processing unit 41 further includes motionestimation unit 42, motion compensation unit 44, partition unit 45,intra prediction processing unit 46, and intra block copy (BC) unit 48.In some implementations, video encoder 20 also includes inversequantization unit 58, inverse transform processing unit 60, and summer62 for video block reconstruction. An in-loop filter, such as adeblocking filter (not shown) may be positioned between summer 62 andDPB 64 to filter block boundaries to remove blockiness artifacts fromreconstructed video. Another in-loop filter (not shown) may also be usedin addition to the deblocking filter to filter the output of summer 62.Further in-loop filtering, such as sample adaptive offset (SAO) andadaptive in-loop filter (ALF) may be applied on the reconstructed CUbefore it is put in the reference picture store and used as reference tocode future video blocks. Video encoder 20 may take the form of a fixedor programmable hardware unit or may be divided among one or more of theillustrated fixed or programmable hardware units.

Video data memory 40 may store video data to be encoded by thecomponents of video encoder 20. The video data in video data memory 40may be obtained, for example, from video source 18. DPB 64 is a bufferthat stores reference video data for use in encoding video data by videoencoder 20 (e.g., in intra or inter predictive coding modes). Video datamemory 40 and DPB 64 may be formed by any of a variety of memorydevices. In various examples, video data memory 40 may be on-chip withother components of video encoder 20, or off-chip relative to thosecomponents.

As shown in FIG. 2, after receiving video data, partition unit 45 withinprediction processing unit 41 partitions the video data into videoblocks. This partitioning may also include partitioning a video frameinto slices, tiles, or other larger coding units (CUs) according to apredefined splitting structures such as quad-tree structure associatedwith the video data. The video frame may be divided into multiple videoblocks (or sets of video blocks referred to as tiles). Predictionprocessing unit 41 may select one of a plurality of possible predictivecoding modes, such as one of a plurality of intra predictive codingmodes or one of a plurality of inter predictive coding modes, for thecurrent video block based on error results (e.g., coding rate and thelevel of distortion). Prediction processing unit 41 may provide theresulting intra or inter prediction coded block to summer 50 to generatea residual block and to summer 62 to reconstruct the encoded block foruse as part of a reference frame subsequently. Prediction processingunit 41 also provides syntax elements, such as motion vectors,intra-mode indicators, partition information, and other such syntaxinformation, to entropy encoding unit 56.

In order to select an appropriate intra predictive coding mode for thecurrent video block, intra prediction processing unit 46 withinprediction processing unit 41 may perform intra predictive coding of thecurrent video block relative to one or more neighboring blocks in thesame frame as the current block to be coded to provide spatialprediction. Motion estimation unit 42 and motion compensation unit 44within prediction processing unit 41 perform inter predictive coding ofthe current video block relative to one or more predictive blocks in oneor more reference frames to provide temporal prediction. Video encoder20 may perform multiple coding passes, e.g., to select an appropriatecoding mode for each block of video data.

In some implementations, motion estimation unit 42 determines the interprediction mode for a current video frame by generating a motion vector,which indicates the displacement of a prediction unit (PU) of a videoblock within the current video frame relative to a predictive blockwithin a reference video frame, according to a predetermined patternwithin a sequence of video frames. Motion estimation, performed bymotion estimation unit 42, is the process of generating motion vectors,which estimate motion for video blocks. A motion vector, for example,may indicate the displacement of a PU of a video block within a currentvideo frame or picture relative to a predictive block within a referenceframe (or other coded unit) relative to the current block being codedwithin the current frame (or other coded unit). The predeterminedpattern may designate video frames in the sequence as P frames or Bframes. Intra BC unit 48 may determine vectors, e.g., block vectors, forintra BC coding in a manner similar to the determination of motionvectors by motion estimation unit 42 for inter prediction, or mayutilize motion estimation unit 42 to determine the block vector.

A predictive block is a block of a reference frame that is deemed asclosely matching the PU of the video block to be coded in terms of pixeldifference, which may be determined by sum of absolute difference (SAD),sum of square difference (SSD), or other difference metrics. In someimplementations, video encoder 20 may calculate values for sub-integerpixel positions of reference frames stored in DPB 64. For example, videoencoder 20 may interpolate values of one-quarter pixel positions,one-eighth pixel positions, or other fractional pixel positions of thereference frame. Therefore, motion estimation unit 42 may perform amotion search relative to the full pixel positions and fractional pixelpositions and output a motion vector with fractional pixel precision.

Motion estimation unit 42 calculates a motion vector for a PU of a videoblock in an inter prediction coded frame by comparing the position ofthe PU to the position of a predictive block of a reference frameselected from a first reference frame list (List 0) or a secondreference frame list (List 1), each of which identifies one or morereference frames stored in DPB 64. Motion estimation unit 42 sends thecalculated motion vector to motion compensation unit 44 and then toentropy encoding unit 56.

Motion compensation, performed by motion compensation unit 44, mayinvolve fetching or generating the predictive block based on the motionvector determined by motion estimation unit 42. Upon receiving themotion vector for the PU of the current video block, motion compensationunit 44 may locate a predictive block to which the motion vector pointsin one of the reference frame lists, retrieve the predictive block fromDPB 64, and forward the predictive block to summer 50. Summer 50 thenforms a residual video block of pixel difference values by subtractingpixel values of the predictive block provided by motion compensationunit 44 from the pixel values of the current video block being coded.The pixel difference values forming the residual vide block may includeluma or chroma difference components or both. Motion compensation unit44 may also generate syntax elements associated with the video blocks ofa video frame for use by video decoder 30 in decoding the video blocksof the video frame. The syntax elements may include, for example, syntaxelements defining the motion vector used to identify the predictiveblock, any flags indicating the prediction mode, or any other syntaxinformation described herein. Note that motion estimation unit 42 andmotion compensation unit 44 may be highly integrated, but areillustrated separately for conceptual purposes.

In some implementations, intra BC unit 48 may generate vectors and fetchpredictive blocks in a manner similar to that described above inconnection with motion estimation unit 42 and motion compensation unit44, but with the predictive blocks being in the same frame as thecurrent block being coded and with the vectors being referred to asblock vectors as opposed to motion vectors. In particular, intra BC unit48 may determine an intra-prediction mode to use to encode a currentblock. In some examples, intra BC unit 48 may encode a current blockusing various intra-prediction modes, e.g., during separate encodingpasses, and test their performance through rate-distortion analysis.Next, intra BC unit 48 may select, among the various testedintra-prediction modes, an appropriate intra-prediction mode to use andgenerate an intra-mode indicator accordingly. For example, intra BC unit48 may calculate rate-distortion values using a rate-distortion analysisfor the various tested intra-prediction modes, and select theintra-prediction mode having the best rate-distortion characteristicsamong the tested modes as the appropriate intra-prediction mode to use.Rate-distortion analysis generally determines an amount of distortion(or error) between an encoded block and an original, unencoded blockthat was encoded to produce the encoded block, as well as a bitrate(i.e., a number of bits) used to produce the encoded block. Intra BCunit 48 may calculate ratios from the distortions and rates for thevarious encoded blocks to determine which intra-prediction mode exhibitsthe best rate-distortion value for the block.

In other examples, intra BC unit 48 may use motion estimation unit 42and motion compensation unit 44, in whole or in part, to perform suchfunctions for Intra BC prediction according to the implementationsdescribed herein. In either case, for Intra block copy, a predictiveblock may be a block that is deemed as closely matching the block to becoded, in terms of pixel difference, which may be determined by sum ofabsolute difference (SAD), sum of squared difference (SSD), or otherdifference metrics, and identification of the predictive block mayinclude calculation of values for sub-integer pixel positions.

Whether the predictive block is from the same frame according to intraprediction, or a different frame according to inter prediction, videoencoder 20 may form a residual video block by subtracting pixel valuesof the predictive block from the pixel values of the current video blockbeing coded, forming pixel difference values. The pixel differencevalues forming the residual video block may include both luma and chromacomponent differences.

Intra prediction processing unit 46 may intra-predict a current videoblock, as an alternative to the inter-prediction performed by motionestimation unit 42 and motion compensation unit 44, or the intra blockcopy prediction performed by intra BC unit 48, as described above. Inparticular, intra prediction processing unit 46 may determine an intraprediction mode to use to encode a current block. To do so, intraprediction processing unit 46 may encode a current block using variousintra prediction modes, e.g., during separate encoding passes, and intraprediction processing unit 46 (or a mode select unit, in some examples)may select an appropriate intra prediction mode to use from the testedintra prediction modes. Intra prediction processing unit 46 may provideinformation indicative of the selected intra-prediction mode for theblock to entropy encoding unit 56. Entropy encoding unit 56 may encodethe information indicating the selected intra-prediction mode in thebitstream.

After prediction processing unit 41 determines the predictive block forthe current video block via either inter prediction or intra prediction,summer 50 forms a residual video block by subtracting the predictiveblock from the current video block. The residual video data in theresidual block may be included in one or more transform units (TUs) andis provided to transform processing unit 52. Transform processing unit52 transforms the residual video data into residual transformcoefficients using a transform, such as a discrete cosine transform(DCT) or a conceptually similar transform.

Transform processing unit 52 may send the resulting transformcoefficients to quantization unit 54. Quantization unit 54 quantizes thetransform coefficients to further reduce bit rate. The quantizationprocess may also reduce the bit depth associated with some or all of thecoefficients. The degree of quantization may be modified by adjusting aquantization parameter. In some examples, quantization unit 54 may thenperform a scan of a matrix including the quantized transformcoefficients. Alternatively, entropy encoding unit 56 may perform thescan.

Following quantization, entropy encoding unit 56 entropy encodes thequantized transform coefficients into a video bitstream using, e.g.,context adaptive variable length coding (CAVLC), context adaptive binaryarithmetic coding (CABAC), syntax-based context-adaptive binaryarithmetic coding (SBAC), probability interval partitioning entropy(PIPE) coding or another entropy encoding methodology or technique. Theencoded bitstream may then be transmitted to video decoder 30, orarchived in storage device 32 for later transmission to or retrieval byvideo decoder 30. Entropy encoding unit 56 may also entropy encode themotion vectors and the other syntax elements for the current video framebeing coded.

Inverse quantization unit 58 and inverse transform processing unit 60apply inverse quantization and inverse transformation, respectively, toreconstruct the residual video block in the pixel domain for generatinga reference block for prediction of other video blocks. As noted above,motion compensation unit 44 may generate a motion compensated predictiveblock from one or more reference blocks of the frames stored in DPB 64.Motion compensation unit 44 may also apply one or more interpolationfilters to the predictive block to calculate sub-integer pixel valuesfor use in motion estimation.

Summer 62 adds the reconstructed residual block to the motioncompensated predictive block produced by motion compensation unit 44 toproduce a reference block for storage in DPB 64. The reference block maythen be used by intra BC unit 48, motion estimation unit 42 and motioncompensation unit 44 as a predictive block to inter predict anothervideo block in a subsequent video frame.

FIG. 3 is a block diagram illustrating an exemplary video decoder 30 inaccordance with some implementations of the present application. Videodecoder 30 includes video data memory 79, entropy decoding unit 80,prediction processing unit 81, inverse quantization unit 86, inversetransform processing unit 88, summer 90, and DPB 92. Predictionprocessing unit 81 further includes motion compensation unit 82, intraprediction unit 84, and intra BC unit 85. Video decoder 30 may perform adecoding process generally reciprocal to the encoding process describedabove with respect to video encoder 20 in connection with FIG. 2. Forexample, motion compensation unit 82 may generate prediction data basedon motion vectors received from entropy decoding unit 80, while intraprediction unit 84 may generate prediction data based onintra-prediction mode indicators received from entropy decoding unit 80.

In some examples, a unit of video decoder 30 may be tasked to performthe implementations of the present application. Also, in some examples,the implementations of the present disclosure may be divided among oneor more of the units of video decoder 30. For example, intra BC unit 85may perform the implementations of the present application, alone, or incombination with other units of video decoder 30, such as motioncompensation unit 82, intra prediction unit 84, and entropy decodingunit 80. In some examples, video decoder 30 may not include intra BCunit 85 and the functionality of intra BC unit 85 may be performed byother components of prediction processing unit 81, such as motioncompensation unit 82.

Video data memory 79 may store video data, such as an encoded videobitstream, to be decoded by the other components of video decoder 30.The video data stored in video data memory 79 may be obtained, forexample, from storage device 32, from a local video source, such as acamera, via wired or wireless network communication of video data, or byaccessing physical data storage media (e.g., a flash drive or harddisk). Video data memory 79 may include a coded picture buffer (CPB)that stores encoded video data from an encoded video bitstream. Decodedpicture buffer (DPB) 92 of video decoder 30 stores reference video datafor use in decoding video data by video decoder 30 (e.g., in intra orinter predictive coding modes). Video data memory 79 and DPB 92 may beformed by any of a variety of memory devices, such as dynamic randomaccess memory (DRAM), including synchronous DRAM (SDRAM),magneto-resistive RAM (MRAM), resistive RAM (RRAM), or other types ofmemory devices. For illustrative purpose, video data memory 79 and DPB92 are depicted as two distinct components of video decoder 30 in FIG.3. But it will be apparent to one skilled in the art that video datamemory 79 and DPB 92 may be provided by the same memory device orseparate memory devices. In some examples, video data memory 79 may beon-chip with other components of video decoder 30, or off-chip relativeto those components.

During the decoding process, video decoder 30 receives an encoded videobitstream that represents video blocks of an encoded video frame andassociated syntax elements. Video decoder 30 may receive the syntaxelements at the video frame level and/or the video block level. Entropydecoding unit 80 of video decoder 30 entropy decodes the bitstream togenerate quantized coefficients, motion vectors or intra-prediction modeindicators, and other syntax elements. Entropy decoding unit 80 thenforwards the motion vectors and other syntax elements to predictionprocessing unit 81.

When the video frame is coded as an intra predictive coded (I) frame orfor intra coded predictive blocks in other types of frames, intraprediction unit 84 of prediction processing unit 81 may generateprediction data for a video block of the current video frame based on asignaled intra prediction mode and reference data from previouslydecoded blocks of the current frame.

When the video frame is coded as an inter-predictive coded (i.e., B orP) frame, motion compensation unit 82 of prediction processing unit 81produces one or more predictive blocks for a video block of the currentvideo frame based on the motion vectors and other syntax elementsreceived from entropy decoding unit 80. Each of the predictive blocksmay be produced from a reference frame within one of the reference framelists. Video decoder 30 may construct the reference frame lists, List 0and List 1, using default construction techniques based on referenceframes stored in DPB 92.

In some examples, when the video block is coded according to the intraBC mode described herein, intra BC unit 85 of prediction processing unit81 produces predictive blocks for the current video block based on blockvectors and other syntax elements received from entropy decoding unit80. The predictive blocks may be within a reconstructed region of thesame picture as the current video block defined by video encoder 20.

Motion compensation unit 82 and/or intra BC unit 85 determinesprediction information for a video block of the current video frame byparsing the motion vectors and other syntax elements, and then uses theprediction information to produce the predictive blocks for the currentvideo block being decoded. For example, motion compensation unit 82 usessome of the received syntax elements to determine a prediction mode(e.g., intra or inter prediction) used to code video blocks of the videoframe, an inter prediction frame type (e.g., B or P), constructioninformation for one or more of the reference frame lists for the frame,motion vectors for each inter predictive encoded video block of theframe, inter prediction status for each inter predictive coded videoblock of the frame, and other information to decode the video blocks inthe current video frame.

Similarly, intra BC unit 85 may use some of the received syntaxelements, e.g., a flag, to determine that the current video block waspredicted using the intra BC mode, construction information of whichvideo blocks of the frame are within the reconstructed region and shouldbe stored in DPB 92, block vectors for each intra BC predicted videoblock of the frame, intra BC prediction status for each intra BCpredicted video block of the frame, and other information to decode thevideo blocks in the current video frame.

Motion compensation unit 82 may also perform interpolation using theinterpolation filters as used by video encoder 20 during encoding of thevideo blocks to calculate interpolated values for sub-integer pixels ofreference blocks. In this case, motion compensation unit 82 maydetermine the interpolation filters used by video encoder 20 from thereceived syntax elements and use the interpolation filters to producepredictive blocks.

Inverse quantization unit 86 inverse quantizes the quantized transformcoefficients provided in the bitstream and entropy decoded by entropydecoding unit 80 using the same quantization parameter calculated byvideo encoder 20 for each video block in the video frame to determine adegree of quantization. Inverse transform processing unit 88 applies aninverse transform, e.g., an inverse DCT, an inverse integer transform,or a conceptually similar inverse transform process, to the transformcoefficients in order to reconstruct the residual blocks in the pixeldomain.

After motion compensation unit 82 or intra BC unit 85 generates thepredictive block for the current video block based on the vectors andother syntax elements, summer 90 reconstructs decoded video block forthe current video block by summing the residual block from inversetransform processing unit 88 and a corresponding predictive blockgenerated by motion compensation unit 82 and intra BC unit 85. Anin-loop filter (not pictured) may be positioned between summer 90 andDPB 92 to further process the decoded video block. The in-loopfiltering, such as deblocking filter, sample adaptive offset (SAO) andadaptive in-loop filter (ALF) may be applied on the reconstructed CUbefore it is put in the reference picture store. The decoded videoblocks in a given frame are then stored in DPB 92, which storesreference frames used for subsequent motion compensation of next videoblocks. DPB 92, or a memory device separate from DPB 92, may also storedecoded video for later presentation on a display device, such asdisplay device 34 of FIG. 1.

In a typical video coding process, a video sequence typically includesan ordered set of frames or pictures. Each frame may include threesample arrays, denoted SL, SCb, and SCr. SL is a two-dimensional arrayof luma samples. SCb is a two-dimensional array of Cb chroma samples.SCr is a two-dimensional array of Cr chroma samples. In other instances,a frame may be monochrome and therefore includes only onetwo-dimensional array of luma samples.

Like the HEVC, the AVS3 standard is built upon the block-based hybridvideo coding framework. The input video signal is processed block byblock (called coding units (CUs)). Different from the HEVC whichpartitions blocks only based on quad-trees, in the AVS3, one coding treeunit (CTU) is split into CUs to adapt to varying local characteristicsbased on quad/binary/extended-quad-tree. Additionally, the concept ofmultiple partition unit type in the HEVC is removed, i.e., theseparation of CU, prediction unit (PU) and transform unit (TU) does notexist in the AVS3. Instead, each CU is always used as the basic unit forboth prediction and transform without further partitions. In the treepartition structure of the AVS3, one CTU is firstly partitioned based ona quad-tree structure. Then, each quad-tree leaf node can be furtherpartitioned based on a binary and extended-quad-tree structure.

As shown in FIG. 4A, video encoder 20 (or more specifically partitionunit 45) generates an encoded representation of a frame by firstpartitioning the frame into a set of coding tree units (CTUs). A videoframe may include an integer number of CTUs ordered consecutively in araster scan order from left to right and from top to bottom. Each CTU isa largest logical coding unit and the width and height of the CTU aresignaled by the video encoder 20 in a sequence parameter set, such thatall the CTUs in a video sequence have the same size being one of128×128, 64×64, 32×32, and 16×16. But it should be noted that thepresent application is not necessarily limited to a particular size. Asshown in FIG. 4B, each CTU may comprise one coding tree block (CTB) ofluma samples, two corresponding coding tree blocks of chroma samples,and syntax elements used to code the samples of the coding tree blocks.The syntax elements describe properties of different types of units of acoded block of pixels and how the video sequence can be reconstructed atthe video decoder 30, including inter or intra prediction, intraprediction mode, motion vectors, and other parameters. In monochromepictures or pictures having three separate color planes, a CTU maycomprise a single coding tree block and syntax elements used to code thesamples of the coding tree block. A coding tree block may be an N×Nblock of samples.

To achieve a better performance, video encoder 20 may recursivelyperform tree partitioning such as binary-tree partitioning, ternary-treepartitioning, quad-tree partitioning or a combination of both on thecoding tree blocks of the CTU and divide the CTU into smaller codingunits (CUs). As depicted in FIG. 4C, the 64×64 CTU 400 is first dividedinto four smaller CU, each having a block size of 32×32. Among the foursmaller CUs, CU 410 and CU 420 are each divided into four CUs of 16×16by block size. The two 16×16 CUs 430 and 440 are each further dividedinto four CUs of 8×8 by block size. FIG. 4D depicts a quad-tree datastructure illustrating the end result of the partition process of theCTU 400 as depicted in FIG. 4C, each leaf node of the quad-treecorresponding to one CU of a respective size ranging from 32×32 to 8×8.Like the CTU depicted in FIG. 4B, each CU may comprise a coding block(CB) of luma samples and two corresponding coding blocks of chromasamples of a frame of the same size, and syntax elements used to codethe samples of the coding blocks. In monochrome pictures or pictureshaving three separate color planes, a CU may comprise a single codingblock and syntax structures used to code the samples of the codingblock. It should be noted that the quad-tree partitioning depicted inFIGS. 4C and 4D is only for illustrative purposes and one CTU can besplit into CUs to adapt to varying local characteristics based onquad/ternary/binary-tree partitions. In the multi-type tree structure,one CTU is partitioned by a quad-tree structure and each quad-tree leafCU can be further partitioned by a binary and ternary tree structure. Asshown in FIG. 4E, there are five splitting/partitioning types in theAVS3, i.e., quaternary partitioning, horizontal binary partitioning,vertical binary partitioning, horizontal extended quad-treepartitioning, and vertical extended quad-tree partitioning.

In some implementations, video encoder 20 may further partition a codingblock of a CU into one or more M×N prediction blocks (PB). A predictionblock is a rectangular (square or non-square) block of samples on whichthe same prediction, inter or intra, is applied. A prediction unit (PU)of a CU may comprise a prediction block of luma samples, twocorresponding prediction blocks of chroma samples, and syntax elementsused to predict the prediction blocks. In monochrome pictures orpictures having three separate color planes, a PU may comprise a singleprediction block and syntax structures used to predict the predictionblock. Video encoder 20 may generate predictive luma, Cb, and Cr blocksfor luma, Cb, and Cr prediction blocks of each PU of the CU.

Video encoder 20 may use intra prediction or inter prediction togenerate the predictive blocks for a PU. If video encoder 20 uses intraprediction to generate the predictive blocks of a PU, video encoder 20may generate the predictive blocks of the PU based on decoded samples ofthe frame associated with the PU. If video encoder 20 uses interprediction to generate the predictive blocks of a PU, video encoder 20may generate the predictive blocks of the PU based on decoded samples ofone or more frames other than the frame associated with the PU.

After video encoder 20 generates predictive luma, Cb, and Cr blocks forone or more PUs of a CU, video encoder 20 may generate a luma residualblock for the CU by subtracting the CU's predictive luma blocks from itsoriginal luma coding block such that each sample in the CU's lumaresidual block indicates a difference between a luma sample in one ofthe CU's predictive luma blocks and a corresponding sample in the CU'soriginal luma coding block. Similarly, video encoder 20 may generate aCb residual block and a Cr residual block for the CU, respectively, suchthat each sample in the CU's Cb residual block indicates a differencebetween a Cb sample in one of the CU's predictive Cb blocks and acorresponding sample in the CU's original Cb coding block and eachsample in the CU's Cr residual block may indicate a difference between aCr sample in one of the CU's predictive Cr blocks and a correspondingsample in the CU's original Cr coding block.

Furthermore, as illustrated in FIG. 4C, video encoder 20 may usequad-tree partitioning to decompose the luma, Cb, and Cr residual blocksof a CU into one or more luma, Cb, and Cr transform blocks. A transformblock is a rectangular (square or non-square) block of samples on whichthe same transform is applied. A transform unit (TU) of a CU maycomprise a transform block of luma samples, two corresponding transformblocks of chroma samples, and syntax elements used to transform thetransform block samples. Thus, each TU of a CU may be associated with aluma transform block, a Cb transform block, and a Cr transform block. Insome examples, the luma transform block associated with the TU may be asub-block of the CU's luma residual block. The Cb transform block may bea sub-block of the CU's Cb residual block. The Cr transform block may bea sub-block of the CU's Cr residual block. In monochrome pictures orpictures having three separate color planes, a TU may comprise a singletransform block and syntax structures used to transform the samples ofthe transform block.

Video encoder 20 may apply one or more transforms to a luma transformblock of a TU to generate a luma coefficient block for the TU. Acoefficient block may be a two-dimensional array of transformcoefficients. A transform coefficient may be a scalar quantity. Videoencoder 20 may apply one or more transforms to a Cb transform block of aTU to generate a Cb coefficient block for the TU. Video encoder 20 mayapply one or more transforms to a Cr transform block of a TU to generatea Cr coefficient block for the TU.

After generating a coefficient block (e.g., a luma coefficient block, aCb coefficient block or a Cr coefficient block), video encoder 20 mayquantize the coefficient block. Quantization generally refers to aprocess in which transform coefficients are quantized to possibly reducethe amount of data used to represent the transform coefficients,providing further compression. After video encoder 20 quantizes acoefficient block, video encoder 20 may entropy encode syntax elementsindicating the quantized transform coefficients. For example, videoencoder 20 may perform Context-Adaptive Binary Arithmetic Coding (CABAC)on the syntax elements indicating the quantized transform coefficients.Finally, video encoder 20 may output a bitstream that includes asequence of bits that forms a representation of coded frames andassociated data, which is either saved in storage device 32 ortransmitted to destination device 14.

After receiving a bitstream generated by video encoder 20, video decoder30 may parse the bitstream to obtain syntax elements from the bitstream.Video decoder 30 may reconstruct the frames of the video data based atleast in part on the syntax elements obtained from the bitstream. Theprocess of reconstructing the video data is generally reciprocal to theencoding process performed by video encoder 20. For example, videodecoder 30 may perform inverse transforms on the coefficient blocksassociated with TUs of a current CU to reconstruct residual blocksassociated with the TUs of the current CU. Video decoder 30 alsoreconstructs the coding blocks of the current CU by adding the samplesof the predictive blocks for PUs of the current CU to correspondingsamples of the transform blocks of the TUs of the current CU. Afterreconstructing the coding blocks for each CU of a frame, video decoder30 may reconstruct the frame.

SAO is a process that modifies the decoded samples by conditionallyadding an offset value to each sample after the application of thedeblocking filter, based on values in look-up tables transmitted by theencoder. SAO filtering is performed on a region basis, based on afiltering type selected per CTB by a syntax element sao-type-idx. Avalue of 0 for sao-type-idx indicates that the SAO filter is not appliedto the CTB, and the values 1 and 2 signal the use of the band offset andedge offset filtering types, respectively. In the band offset modespecified by sao-type-idx equal to 1, the selected offset value directlydepends on the sample amplitude. In this mode, the full sample amplituderange is uniformly split into 32 segments called bands, and the samplevalues belonging to four of these bands (which are consecutive withinthe 32 bands) are modified by adding transmitted values denoted as bandoffsets, which can be positive or negative. The main reason for usingfour consecutive bands is that in the smooth areas where bandingartifacts can appear, the sample amplitudes in a CTB tend to beconcentrated in only few of the bands. In addition, the design choice ofusing four offsets is unified with the edge offset mode of operationwhich also uses four offset values. In the edge offset mode specified bysao-type-idx equal to 2, a syntax element sao-eo-class with values from0 to 3 signals whether a horizontal, vertical or one of two diagonalgradient directions is used for the edge offset classification in theCTB.

FIG. 5 is a block diagram depicting the four gradient patterns used inSAO in accordance with some implementations of the present disclosure.The four gradient patterns 502, 504, 506, and 508 are for the respectivesao-eo-class in the edge offset mode. Sample labelled “p” indicates acenter sample to be considered. Two samples labeled “n0” and “n1”specify two neighboring samples along the (a) horizontal(sao-eo-class=0), (b) vertical (sao-eo-class=1), (c) 135° diagonal(sao-eo-class=2), and (d) 45° (sao-eo-class=3) gradient patterns. Eachsample in the CTB is classified into one of five EdgeIdx categories bycomparing the sample value p located at some position with the values n0and n1 of two samples located at neighboring positions as shown in FIG.5. This classification is done for each sample based on decoded samplevalues, so no additional signaling is required for the EdgeIdxclassification. Depending on the EdgeIdx category at the sampleposition, for EdgeIdx categories from 1 to 4, an offset value from atransmitted look-up table is added to the sample value. The offsetvalues are always positive for categories 1 and 2 and negative forcategories 3 and 4. Thus the filter generally has a smoothing effect inthe edge offset mode. Table 1 below illustrates a sample EdgeIdxcategories in SAO edge classes.

TABLE 1 A sample EdgeIdx categories in SAO edge classes. EdgeIdxCondition Meaning 0 Cases not listed below Monotonic area 1 p < n₀ and p< n₁ Local min 2 p < n₀ and p = n₁ or p < n₁ and p = n₀ Edge 3 p > n₀and p = n₁ or p > n₁ and p = n₀ Edge 4 p > n₀ and p > n₁ Local max

For SAO types 1 and 2, a total of four amplitude offset values aretransmitted to the decoder for each CTB. For type 1, the sign is alsoencoded. The offset values and related syntax elements such assao-type-idx and sao-eo-class are determined by the encoder—typicallyusing criteria that optimize rate-distortion performance. The SAOparameters can be indicated to be inherited from the left or above CTBusing a merge flag to make the signaling efficient. In summary, SAO is anonlinear filtering operation which allows additional refinement of thereconstructed signal, and it can enhance the signal representation inboth smooth areas and around edges.

In some embodiments, methods and systems are disclosed herein to improvethe coding efficiency or reduce the complexity of Sample Adaptive Offset(SAO) by introducing cross-component information. SAO is used in theHEVC, VVC, AVS2 and AVS3 standards. Although the existing SAO design inthe HEVC, VVC, AVS2 and AVS3 standards is used as the basic SAO methodin the following descriptions, to a person skilled in the art of videocoding, the cross-component methods described in the disclosure can alsobe applied to other loop filter designs or other coding tools with thesimilar design spirits. For example, in the AVS3 standard, SAO isreplaced by a coding tool called Enhanced Sample Adaptive Offset (ESAO).However, the CCSAO disclosed herein can also be applied in parallel withESAO. In another example, CCSAO can be applied in parallel withConstrained Directional Enhancement Filter (CDEF) in the AV1 standard.

For the existing SAO design in the HEVC, VVC, AVS2 and AVS3 standards,the luma Y, chroma Cb and chroma Cr sample offset values are decidedindependently. That is, for example, the current chroma sample offset isdecided by only the current and neighboring chroma sample values,without taking collocated or neighboring luma samples intoconsideration. However, luma samples preserve more original picturedetail information than chroma samples, and they can benefit thedecision of the current chroma sample offset. Furthermore, since chromasamples usually lose high frequency details after color conversion fromRGB to YCbCr, or after quantization and deblocking filter, introducingluma samples with high frequency detail preserved for chroma offsetdecision can benefit the chroma sample reconstruction. Hence, furthergain can be expected by exploring cross-component correlation, forexample, by using the methods and systems of Cross-Component SampleAdaptive Offset (CCSAO).

FIG. 6A is a block diagram illustrating the system and process of CCSAOaccording to some implementations of the present disclosure. The lumasamples after luma deblocking filter (DBF Y) is used to determineadditional offsets for chroma Cb and Cr after SAO Cb and SAO Cr. Forexample, the current chroma sample 602 is first classified usingcollocated 604 and neighboring (white) luma samples 606, and thecorresponding CCSAO offset value of the corresponding class is added tothe current chroma sample value.

In some embodiments, CCSAO can also be applied in parallel with othercoding tools, for example, ESAO in the AVS standard, or CDEF in the AV1standard. FIG. 6B is a block diagram illustrating the system and processof CCSAO applied in parallel with ESAO in the AVS standard according tosome implementations of the present disclosure.

In some embodiments, the current chroma sample classification is reusingthe SAO type (EO or BO), class, and category of the collocated lumasample. The corresponding CCSAO offset can be signaled or derived fromthe decoder itself. For example, let h_Y be the collocated luma SAOoffset, h_Cb and h_Cr be the CCSAO Cb and Cr offset, respectively. h_Cb(or h_Cr)=w*h_Y where w can be selected in a limited table. For example,+−¼, +−½, 0, +−1, +−2, +−4 . . . etc., where |w| only includes thepower-of-2 values.

In some embodiments, the comparison score [−8, 8] of the collocated lumasamples (Y0) and neighboring 8 luma samples are used, which yields 17classes in total.

Initial  Class = 0Loop  over  neighboring  8  luma  samples  (Yi, i = 1  to  8)$\begin{matrix}{if} & {{Y0} > {Yi}} & {{Class}+=1} \\{{else}\mspace{14mu}{if}} & {{Y0} < {Yi}} & {{Class}-=1}\end{matrix}$

In some embodiments, the abovementioned classification methods can becombined. For example, comparison score combined with SAO BO (32 bandsclassification) is used to increase diversity, which yields 17*32classes in total. In some embodiments, the Cb and Cr can use the sameclass to reduce the complexity or saving bits.

FIG. 7 is a block diagram illustrating a sample process using CCSAO inaccordance with some implementations of the present disclosure.Specifically, FIG. 7 shows the input of CCSAO can introduce the input ofvertical and horizontal DBF, to simplify the class determination, orincrease flexibility. For example, let Y0_DBF_V, Y0_DBF_H, and Y0 becollocated luma samples at the input of DBF_V, DBF_H, and SAO,respectively. Yi_DBF_V, Yi_DBF_H, and Yi are neighbouring 8 luma samplesat the input of DBF_V, DBF_H, and SAO, respectively, where i=1 to 8.

Max Y0=max(Y0_DBF_V,Y0_DBF_H,Y0_DBF)

Max Yi=max(Yi_DBF_V,Yi_DBF_H,Yi_DBF)

And feed max Y0 and max Yi to CCSAO classification.

FIG. 8 is a block diagram illustrating that CCSAO process is interleavedto vertical and horizontal DBF in accordance with some implementationsof the present disclosure. In some embodiments, CCSAO blocks in FIGS. 6,7 and 8 can be selective. For example, using Y0_DBF_V and Yi_DBF_V forthe first CCSAO V, which applies the same sample processing as in FIG.6, while using the input of DBF_V luma samples as CCSAO input.

In some embodiments, CCSAO syntax implemented is shown in Table 2 below.

TABLE 2 An example of CCSAO syntax Level Syntax element Meaning SPScc_sao_enabled_flag whether CCSAO is enabled in the sequence SHslice_cc_sao_cb_flag whether CCSAO is enabled for slice_cc_sao_cr_flagCb or Cr CTU cc_sao_merge_left_flag whether CCSAO offset is merged fromcc_sao_merge_up_flag the left or up CTU CTU cc_sao_class_idx CCSAO classindex of this CTU CTU cc_sao_offset_sign_flag CCSAO Cb and Cr offsetvalues of cc_sao_offset_abs this CTU class

In some embodiments, for signaling CCSAO Cb and Cr offset values, if oneadditional chroma offset is signaled, the other chroma component offsetcan be derived by plus or minus sign, or weighting to save bitsoverhead. For example, let h_Cb and h_Cr be the offset of CCSAO Cb andCr, respectively. With explicit signaling w, wherein w=+−|w| withlimited |w| candidates, h_Cr can be derived from h_Cb without explicitsignaling h_Cr itself.

h_Cr=w*h_Cb

FIG. 9 is a flowchart illustrating an exemplary process 900 of decodingvideo signal using cross-component correlation in accordance with someimplementations of the present disclosure.

The video decoder 30, receives the video signal that includes a firstcomponent and a second component (910). In some embodiments, the firstcomponent is a luma component, and the second component is a chromacomponent of the video signal.

The video decoder 30 also receives a plurality of offsets associatedwith the second component (920).

The video decoder 30 then utilizes a characteristic measurement of thefirst component to obtain a classification category associated with thesecond component (930). For example, in FIG. 6, the current chromasample 602 is first classified using collocated 604 and neighboring(white) luma samples 606, and the corresponding CCSAO offset value isadded to the current chroma sample.

The video decoder 30 further selects a first offset from the pluralityof offsets for the second component according to the classificationcategory (940).

The video decoder 30 additionally modifies the second component based onthe selected first offset (950).

In some embodiments, utilizing the characteristic measurement of thefirst component to obtain the classification category associated withthe second component (930) includes: utilizing a respective sample ofthe first component to obtain a respective classification category of arespective each sample of the second component, wherein the respectivesample of the first component is a respective collocated sample of thefirst component to the respective each sample of the second component.For example, the current chroma sample classification is reusing the SAOtype (EO or BO), class, and category of the collocated luma sample.

In some embodiments, utilizing the characteristic measurement of thefirst component to obtain the classification category associated withthe second component (930) includes: utilizing a respective sample ofthe first component to obtain a respective classification category of arespective each sample of the second component, wherein the respectivesample of the first component is reconstructed before being deblocked oris reconstructed after being deblocked. In some embodiment, the firstcomponent is being deblocked at a deblocking filter (DBF). In someembodiment, the first component is being deblocked at a luma deblockingfilter (DBF Y). For example, alternative to FIG. 6 or 7, the CCSAO inputcan also be before DBF Y.

In some embodiments, the characteristic measurement is derived bydividing the range of sample values of the first component into severalbands and selecting a band based on the intensity value of a sample inthe first component. In some embodiments, the characteristic measurementis derived from Band Offset (BO).

In some embodiments, the characteristic measurement is derived based onthe direction and strength of the edge information of a sample in thefirst component. In some embodiments, the characteristic measurement isderived from Edge Offset (EO).

In some embodiments, modifying the second component (950) comprisesdirectly adding the selected first offset to the second component. Forexample, the corresponding CCSAO offset value is added to the currentchroma component sample.

In some embodiments, modifying the second component (950) comprisesmapping the selected first offset to a second offset and adding themapped second offset to the second component. For example, for signalingCCSAO Cb and Cr offset values, if one additional chroma offset issignaled, the other chroma component offset can be derived by using aplus or minus sign, or weighting to save bits overhead.

In some embodiments, receiving the video signal (910) comprisesreceiving a syntax element that indicates whether the method of decodingvideo signal using CCSAO is enabled for the video signal in the SequenceParameter Set (SPS). In some embodiments, cc_sao_enabled_flag indicateswhether CCSAO is enabled in the sequence level.

In some embodiments, receiving the video signal (910) comprisesreceiving a syntax element that indicates whether the method of decodingvideo signal using CCSAO is enabled for the second component on theslice level. In some embodiments, slice_cc_sao_cb_flag orslice_cc_sao_cr_flag indicates whether CCSAO is enabled in therespective slice for Cb or Cr.

In some embodiments, receiving the plurality of offsets associated withthe second component (920) comprises receiving different offsets fordifferent Coding Tree Units (CTUs). In some embodiments, for a CTU,cc_sao_offset_sign_flag indicates a sign for an offset, andcc_sao_offset_abs indicates the CCSAO Cb and Cr offset values of thecurrent CTU.

In some embodiments, receiving the plurality of offsets associated withthe second component (920) comprises receiving a syntax element thatindicates whether the received offsets of a CTU are the same as that ofone of a neighboring CTU of the CTU, wherein the neighboring CTU iseither a left or a top neighboring CTU. For example,cc_sao_merge_up_flag indicates whether CCSAO offset is merged from theleft or up CTU.

In some embodiments, the video signal further includes a third componentand the method of decoding the video signal using CCSAO furtherincludes: receiving a second plurality of offsets associated with athird component; utilizing the characteristic measurement of the firstcomponent to obtain a second classification category associated with thethird component; selecting a third offset from the second plurality ofoffsets for the third component according to the second classificationcategory; and modifying the third component based on the selected thirdoffset.

FIG. 11 is a block diagram of a sample process illustrating that besidesluma, the other cross-component collocated (1102) and neighboring(white) chroma samples are also fed into CCSAO classification inaccordance with some implementations of the present disclosure. FIG. 6A,6B and FIG. 11 show the input of CCSAO classification. In FIG. 11,current chroma sample is 1104, the cross-component collocated chromasample is 1102, and the collocated luma sample is 1106.

In some embodiments, a classifier example (C0) uses the collocated lumasample value (Y0) for classification. Let band_num be the number ofequally divided bands of luma dynamic range, and bit_depth be thesequence bit depth, the class index for the current chroma sample is:

Class (C0)=(Y0*band_num)>>bit_depth

Some band_num and bit_depth examples are listed below in Table 3. Table3 shows three classification examples when the number of bands isdifferent for each of the classification examples.

TABLE 3 Exemplary band_num and bit_depth for each class index. band_num16 bit_depth 10 Class Y0 0 0 63 1 64 127 2 128 191 3 192 255 4 256 319 5320 383 6 384 447 7 448 511 8 512 575 9 576 639 10 640 703 11 704 767 12768 831 13 832 895 14 896 959 15 960 1023 band_num 7 bit_depth 10 ClassY0 0 0 145 1 146 292 2 293 438 3 439 584 4 585 730 5 731 877 6 878 1023band_num 7 bit_depth 8 Class Y0 0 0 36 1 37 72 2 73 109 3 110 145 4 146182 5 183 218 6 219 255

In some embodiments, a classifier (or class index) uses different lumasample position for C0 classification. FIG. 10 is a block diagramshowing a classifier using different luma sample position for C0classification in accordance with some implementations of the presentdisclosure, for example, using the neighboring Y7 but not Y0 for C0classification.

In some embodiments, different classifiers can be switched in SequenceParameter Set (SPS)/Adaptation parameter set (APS)/Picture parameter set(PPS)/Picture header (PH)/Slice header (SH)/Coding tree unit(CTU)/Coding unit (CU) levels. For example, in FIG. 10, using Y0 forPOC0 but using Y7 for POC1, as shown in Table 4 below.

TABLE 4 Different classifiers are applied to different pictures POCClassifier C0 band_num Total classes 0 C0 using Y0 position 8 8 1 C0using Y7 position 8 8

In some embodiments, the C0 position and C0 band_num can be combined andswitched in SPS/APS/PPS/PH/SH/CTU/CU levels. Different combinations canbe different classifiers as shown in Table 5 below.

TABLE 5 Different classifier and band number combinations are applied todifferent pictures POC Classifier C0 band_num Total classes 0 C0 usingY0 position 16 16 1 C0 using Y7 position 8 8

In some embodiments, the collocated luma sample value (Y0) is replacedby a value (Yp) obtained by weighing collocated and neighboring lumasamples. FIG. 12 illustrates exemplary classifiers by replacing thecollocated luma sample value with a value obtained by weighingcollocated and neighboring luma samples in accordance with someimplementations of the present disclosure. The collocated luma samplevalue (Y0) can be replaced by a phase corrected value (Yp) obtained byweighing neighboring luma samples. Different Yp can be a differentclassifier.

In some embodiments, different Yp is applied on different chroma format.For example, in FIG. 12, the Yp of (a) is used for the 420 chromaformat, the Yp of (b) is used for the 422 chroma format, and Y0 is usedfor the 444 chroma format.

In some embodiments, another classifier (C1) is the comparison score[−8, 8] of the collocated luma samples (Y0) and neighboring 8 lumasamples, which yields 17 classes in total as shown below.

Initial  Class  (C 1) = 0, Loop  over  neighboring  8  luma  sample  (Yi, i = 1  to  8)$\begin{matrix}{if} & {{Y0} > {Yi}} & {{Class}+=1} \\{{else}\mspace{14mu}{if}} & {{Y0} < {Yi}} & {{Class}{- =}1}\end{matrix}$

In some embodiments, a variation (C1′) only counts comparison score [0,8], and this yields 8 classes. (C1, C1′) is a classifier group and aPH/SH level flag can be signaled to switch between C1 and C1′.

Initial  Class(C 1^(′)) = 0, Loop  over  neighboring  8  luma  samples  (Yi, i = 1  to  8)$\begin{matrix}{if} & {{Y0} > {Yi}} & {{Class}+=1}\end{matrix}$

In some embodiments, different classifiers are combined to yield ageneral classifier. For example, for different pictures (different POCvalues), different classifiers are applied as shown in Table 6 below.

TABLE 6 Different general classifiers are applied to different picturesPOC Classifier C0 band_num Total classes 0 combine C0 and C1 16 16*17 1combine C0 and C1' 16 16*9 2 combine C0 and C1 7 7*17

In some embodiments, a classifier example (C2) uses the difference (Yn)of collocated and neighboring luma samples. FIG. 12 (c) shows an exampleof Yn, which has a dynamic range of [−1024, 1023] when bit depth is 10.Let C2 band_num be the number of equally divided bands of Yn dynamicrange,

Class (C2)=(Yn+(1<<bit_depth)*band_num)>>(bit_depth+1).

In some embodiments, C0 and C2 are combined to yield a generalclassifier. For example, for different pictures (different POC),different classifiers are applied as shown in Table 7 below.

TABLE 7 Different general classifiers are applied to different picturesPOC Classifier C0 band_num C2 band_num Total classes 0 combine C0 and C216 16 16*17 1 combine C0 and C2 8 7 8*7

In some embodiments, all above mentioned classifiers (C0, C1, C1′, C2)are combined. For example, for different pictures (different POCs),different classifiers are applied as shown in Table 8 below.

TABLE 8 Different general classifiers are applied to different picturesTotal POC Classifier C0 band_num C2 band_num classes 0 combine C0, C1and C2 4 4 4*17*4 1 combine C0, C1' and C2 6 4 6*9*4

In some embodiments, plural classifiers are used in the same POC. Thecurrent frame is divided by several regions, and each region uses thesame classifier. For example, 3 different classifiers are used in POC0,and which classifier (0, 1, or 2) is used is signaled in CTU level asshown in Table 9 below.

TABLE 9 Different general classifiers are applied to different regionsin the same picture POC Classifier C0 band_num Region 0 C0 using Y0position 16 0 0 C0 using Y0 position 8 1 0 C0 using Y1 position 8 2

In some embodiments, the classifiers applied to Cb and Cr are different.The Cb and Cr offsets for all classes can be signaled separately. Forexample, different signaled offsets are applied to different chromacomponents as shown in Table 10 below.

TABLE 10 The Cb and Cr offsets for all classes can be signaledseparately POC Component Classifier C0 Total classes Signaled offsets 0Cb C0 16 16 16 0 Cr C0 5 5 5

In some embodiments, the max offset value is fixed or signaled inSequence Parameter Set (SPS)/Adaptation parameter set (APS)/Pictureparameter set (PPS)/Picture header (PH)/Slice header (SH). For example,the max offset is between [−15, 15].

In some embodiments, the offset signaling can use Differentialpulse-code modulation (DPCM). For example, offsets {3, 3, 2, 1, −1} canbe signaled as {3, 0, −1, −1, −2}.

In some embodiments, the offsets can be stored in APS or a memory bufferfor the next picture/slice reuse. An index can be signaled to indicatewhich stored previous frame offsets are used for the current picture.

In some embodiments, the classifiers of Cb and Cr are the same. The Cband Cr offsets for all classes can be signaled jointly, for example, asshown in Table 11 below.

TABLE 11 The Cb and Cr offsets for all classes can be signaled jointlyPOC Component Classifier C0 Total classes Signaled offsets 0 Cb and CrC0 8 8 8

In some embodiments, the classifier of Cb and Cr can be the same. The Cband Cr offsets for all classes can be signaled jointly, with a sign flagdifference, for example, as shown in Table 12 below. According to Table12, when Cb offsets are (3, 3, 2, −1), the derived Cr offsets are (−3,−3, −2, 1).

TABLE 12 The Cb and Cr offsets for all classes can be signaled jointlywith a sign flag difference C0 Total Signaled Signaled POC ComponentClassifier band_num classes offsets sign flag 0 Cb and Cr C0 4 4 4: 1:(−) (3, 3, 2, −1)

In some embodiments, the sign flag can be signaled for each class. forexample, as shown in Table 13 below. According to Table 13, when Cboffsets are (3, 3, 2, −1), the derived Cr offsets are (−3, 3, 2, 1)according to the respective signed flag.

TABLE 13 The Cb and Cr offsets for all classes can be signaled jointlywith a sign flag signaled for each class C0 Total Signaled Signaled POCComponent Classifier band_num classes offsets sign flag 0 Cb and Cr C0 44 4: 1: (3, 3, (−, +, 2, −1) +, −)

In some embodiments, the classifiers of Cb and Cr can be the same. TheCb and Cr offsets for all classes can be signaled jointly, with a weightdifference, for example, as shown in Table 14 below. The weight (w) canbe selected in a limited table, for example, +−¼, +−½, 0, +−1, +−2, +−4. . . etc., where |w| only includes the power-of-2 values. According toTable 14, when Cb offsets are (3, 3, 2, −1), the derived Cr offsets are(−6, −6, −4, 2) according to the respective signed flag.

TABLE 14 The Cb and Cr offsets for all classes can be signaled jointlywith a weight difference C0 Total Signaled Signaled POC ComponentClassifier band_num classes offsets weight 0 Cb and Cr C0 4 4 4: −2 (3,3, 2, −1)

In some embodiments, the weight can be signaled for each class. forexample, as shown in Table 15 below. According to Table 15, when Cboffsets are (3, 3, 2, −1), the derived Cr offsets are (−6, 12, 0, −1)according to the respective signed flag.

TABLE 15 The Cb and Cr offsets for all classes can be signaled jointlywith a weight signaled for each class C0 Total Signaled Signaled POCComponent Classifier band_num classes offsets weight 0 Cb and Cr C0 4 44: 4: (3, 3, (−2, 4, 2, −1) 0, 1)

In some embodiments, if plural classifiers are used in the same POC,different offset sets are signaled separately or jointly.

In some embodiments, the previously decoded offsets can be stored foruse of future frames. An index can be signaled to indicate whichpreviously decoded offsets set is used for the current frame, to reduceoffsets signaling overhead. For example, POC0 offsets can be reused byPOC2 with signaling offsets set idx=0 as shown in Table 16 below.

TABLE 16 An index can be signaled to indicate which previously decodedoffsets set is used for the current frame Stored Com- Classi- C0 TotalSignaled offset POC ponent fier band_num classes offsets set idx 0 Cb C04 4 4: 0 (3, 3, 2, −1) 0 Cr C0 4 4 4: 0 (−2, 1, 0, 1) 1 Cb C0 4 4 4: 1(0, 0, 1, −1) 1 Cr C0 4 4 4: 1 (1, 2, 0, 1) 2 Cb C0 4 4 Reuse Signal idx= offsets 0 (3, 3, 2, −1) 2 Cr C0 4 4 Reuse Signal idx = offsets 0 (−2,1, 0, 1)

In some embodiments, the reuse offsets set idx for Cb and Cr can bedifferent, For example, as shown in Table 17 below.

TABLE 17 An index can be signaled to indicate which previously decodedoffsets set is used Stored Com- Classi- C0 Total Signaled offset POCponent fier band_num classes offsets set idx 0 Cb C0 4 4 4: 0 (3, 3, 2,−1) 0 Cr C0 4 4 4: 0 (−2, 1, 0, 1) 1 Cb C0 4 4 4: 1 (0, 0, 1, −1) 1 CrC0 4 4 4: 1 (1, 2, 0, 1) 2 Cb C0 4 4 Reuse Signal idx = offsets 0 (3, 3,2, −1) 2 Cr C0 4 4 Reuse Signal idx = offsets 1 (1, 2, 0, 1)

for the current frame, and the index can be different for Cb and Crcomponents.

In some embodiments, a sample processing is described below. Let R(x, y)be the input chroma sample value before CCSAO, R′(x, y) be the outputchroma sample value after CCSAO:

offset=ccsao_offset[class_index of R(x,y)]

R′(x,y)=Clip3(0,(1<<bit_depth)−1,R(x,y)+offset)

According the above equations, each chroma sample value R(x, y) isclassified using the indicated classifier of the current picture. Thecorresponding offset of the derived class index is added to each chromasample value R(x, y). A clip function Clip 3 is applied to the (R(x,y)+offset) to make the output chroma sample value R′(x, y) within thebit depth dynamic range, for example, range 0 to (1<<bit_depth)−1.

In some embodiments, a boundary processing is described below. If any ofthe collocated and neighboring luma samples used for classification isoutside the current picture, CCSAO is not applied on the current chromasample. FIG. 13A is a block diagram illustrating CCSAO is not applied onthe current chroma sample if any of the collocated and neighboring lumasamples used for classification is outside the current picture inaccordance with some implementations of the present disclosure. Forexample, in FIG. 13A(a), if a classifier is used, CCSAO is not appliedon the left 1 column chroma components of the current picture. Forexample, if C1′ is used, CCSAO is not applied on the left 1 column andthe top 1 row chroma components of the current picture, as shown in FIG.13A(b).

FIG. 13B is a block diagram illustrating CCSAO is applied on the currentchroma sample if any of the collocated and neighboring luma samples usedfor classification is outside the current picture in accordance withsome implementations of the present disclosure. In some embodiments, avariation is, if any of the collocated and neighboring luma samples usedfor classification is outside the current picture, the missed samplesare used repetitively as shown in FIG. 13B(a), or the missed samples aremirror padded to create samples for classification as shown in FIG.13A(b), and CCSAO can be applied on the current chroma samples.

In some embodiments, CCSAO syntax implemented is shown in Table 18below. In AVS3, the term patch is similar with slice, and patch headeris similar with the slice header. FLC stands for fixed length code. TUstands for truncated unary code. EGk stands for exponential-golomb codewith order k, where k can be fixed.

TABLE 18 A exemplary CCSAO syntax Level Syntax element BinarizationMeaning SPS cc_sao_enabled_flag FLC whether CCSAO is enabled in thesequence PH/SH ph_cc_sao_cb_flag FLC whether CCSAO is enabledph_cc_sao_cr_flag in this picture/slice for Cb/Cr PH/SHph_cc_sao_stored_offsets_set_idx FLC which previously decoded offsetsset is used SPS/APS/ ph_cc_sao_cb_band_num_minus1 FLC adaptively changedband PPS/PH/ ph_cc_sao_cr_band_num_minus1 numbers for classificationSH/CTU SPS/APS/ ph_cc_sao_cb_type FLC Indicating classifier type PPS/PH/ph_cc_sao_cr_type 1. CCSAO C1 classifier SH/CTU luma type (C1 or C1′) 2.CCSAO C0 classifier luma position (Y0, Y1 . . . etc.) 3. CCSAO weightedC0 classifier (Yp, Yn . . . ) SPS/APS/ cc_sao_cb_offset_sign_flag FLCCCSAO Cb and Cr offset PPS/PH/ cc_sao_cb_offset_abs TU or EGk values ofeach class SH/CTU cc_sao_cr_offset_sign_flag FLC cc_sao_cr_offset_abs TUor EGk CTU ctb_cc_sao_cb_flag CABAC, 1 whether CCSAO is enabledctb_cc_sao_cr_flag or 2 (up & for the current Cb or Cr left) CTBcontexts CTU cc_sao_cb_merge_left_flag CABAC whether CCSAO offset iscc_sao_cb_merge_up_ flag merged from the left or upcc_sao_cr_merge_left_flag CTU cc_sao_cr_merge_up_flag

If a higher-level flag is off, the lower level flags can be inferredfrom the off state of the flag and do not need to be signaled. Forexample, if ph_cc_sao_cb_flag is false in this picture,ph_cc_sao_cb_band_num minus1, ph_cc_sao_cb_luma_type,cc_sao_cb_offset_sign_flag, cc_sao_cb_offset_abs, ctb_cc_sao_cb_flag,cc_sao_cb_merge_left_flag, and cc_sao_cb_merge_up_flag are not presentand inferred to be false.

FIG. 14 is a flowchart illustrating an exemplary process 1400 ofdecoding video signal using cross-component correlation in a pictureframe in accordance with some implementations of the present disclosure.

The video decoder 30 (as shown in FIG. 3), receives, the video signalthat includes a first component and a second component in a firstpicture frame (1410). In some embodiments, the first component is a lumacomponent, and the second component is a chroma component.

The video decoder 30 also receives, from the video signal, a pluralityof sample offsets associated with the second component in the firstpicture frame (1420).

The video decoder 30 derives a first classifier for the second componentfrom a first set of one or more samples of the first component relativeto each sample of the second component (1430).

The video decoder 30 then selects a first sample offset from theplurality of sample offsets for the second component according to thefirst classifier (1440).

The video decoder 30 additionally modifies the second component based onthe first sample offset in the first picture frame (1450).

In some embodiments, the video decoder 30 further derives a secondclassifier for the second component in a second region in the firstpicture frame from a second set of one or more samples of the firstcomponent relative to each sample of the second component; selects asecond sample offset from the plurality of sample offsets for the secondcomponent according to the second classifier; and modifies the secondcomponent based on the second sample offset in the second region in thefirst picture frame. The video decoder 30 derives the first classifierfor the second component in the first picture frame in a first region inthe first picture frame.

In some embodiments, the step of selecting the first sample offset fromthe plurality of sample offsets for the second component according tothe first classifier (1440) comprises: dividing a dynamic range ofvalues of the first set of one or more samples of the first componentinto a number of bands according to the first classifier; and selectingthe first sample offset from the plurality of sample offsets for thesecond component corresponding to a first selected band from the numberof bands of the first component.

In some embodiments, the first classifier is a combination of a firstset of one or more classifiers, and the second classifier is acombination of a second set of one or more classifiers.

In some embodiments, the first picture frame is divided into a pluralityof regions, and a different classifier is used for each of the pluralityof regions. In some embodiments, the video decoder 30 further receives asyntax element at CTU level to indicate which classifier is used foreach of the plurality of regions in the first picture frame.

In some embodiments, the video decoder 30 further receives, the videosignal that includes the first component and the second component in asecond picture frame; and the video decoder 30 modifies the secondcomponent in the second picture frame based on an offset set indexindicating which previously decoded offset set in the first pictureframe stored in memory is used for the second picture frame. Forexample, if the first picture frame uses 4 offset sets, the secondpicture frame can reuse the 4 offset sets with idx=0-3. An offset setcan include the offset for each of the bands of sample values.

In some embodiments, the video decoder 30 further receives, the videosignal that includes the first component and the second component in asecond picture frame; the video decoder 30 receives, from the videosignal, a plurality of sample offsets associated with the secondcomponent in the second picture frame; the video decoder 30 derives asecond classifier for the second component from a second set of one ormore samples of the first component relative to each sample of thesecond component; the video decoder 30 selects a second sample offsetfrom the plurality of sample offsets for the second component accordingto the second classifier; and the video decoder 30 modifies the secondcomponent based on the second sample offset in the second picture frame.In some embodiments, the first set of one or more sample values isdefined differently from the second set of one or more sample values.

In some embodiments, the definition of the first set of one or moresamples of the first component relative to each sample of the secondcomponent to derive the first classifier is switched in one or more ofSPS, APS, PPS, PH, SH, CTU, and CU level.

In some embodiments, the definition of the first set of one or moresamples of the first component relative to each sample of the secondcomponent to derive the first classifier is a position of the first setof one or more samples and a number of divided bands of dynamic range ofvalues of the first set of one or more samples of the first component.

In some embodiments, the video decoder 30 further receives, the videosignal that includes the first component and the second component in athird picture frame; and the video decoder modifies the second componentin the third picture frame based on an offset set index indicating whichpreviously decoded offset set in other picture frames is used for thethird picture frame. In some embodiments, the first offset set is storedin memory identified by a first offset set index and the second offsetset is stored in the memory identified by a second offset set index fornext use.

In some embodiments, the video decoder 30 further receives, the videosignal that includes the first component and the second component in athird picture frame; and modifies the second component in the thirdpicture frame based on an offset set index indicating which previouslydecoded offset set in other picture frames is used for the third pictureframe. In some embodiments, the second component comprises a Cbcomponent and a Cr component, and the offset set indexes used for the Cbcomponent and the Cr component are different in the third picture frame.

In some embodiments, the first set of one or more samples of the firstcomponent relative to each sample of the second component is acollocated sample of the first component relative to each sample of thesecond component.

In some embodiments, the first set of one or more samples of the firstcomponent relative to each sample of the second component is aneighboring sample of the first component relative to each sample of thesecond component.

In some embodiments, receiving the video signal (1410) comprisesreceiving a syntax element that indicates whether Cross-component SampleAdaptive Offset (CCSAO) is enabled for the video signal.

Further embodiments also include various subsets of the aboveembodiments combined or otherwise re-arranged in various otherembodiments.

In one or more examples, the functions described may be implemented inhardware, software, firmware, or any combination thereof. If implementedin software, the functions may be stored on or transmitted over, as oneor more instructions or code, a computer-readable medium and executed bya hardware-based processing unit. Computer-readable media may includecomputer-readable storage media, which corresponds to a tangible mediumsuch as data storage media, or communication media including any mediumthat facilitates transfer of a computer program from one place toanother, e.g., according to a communication protocol. In this manner,computer-readable media generally may correspond to (1) tangiblecomputer-readable storage media which is non-transitory or (2) acommunication medium such as a signal or carrier wave. Data storagemedia may be any available media that can be accessed by one or morecomputers or one or more processors to retrieve instructions, codeand/or data structures for implementation of the implementationsdescribed in the present application. A computer program product mayinclude a computer-readable medium.

The terminology used in the description of the implementations herein isfor the purpose of describing particular implementations only and is notintended to limit the scope of claims. As used in the description of theimplementations and the appended claims, the singular forms “a,” “an,”and “the” are intended to include the plural forms as well, unless thecontext clearly indicates otherwise. It will also be understood that theterm “and/or” as used herein refers to and encompasses any and allpossible combinations of one or more of the associated listed items. Itwill be further understood that the terms “comprises” and/or“comprising,” when used in this specification, specify the presence ofstated features, elements, and/or components, but do not preclude thepresence or addition of one or more other features, elements,components, and/or groups thereof.

It will also be understood that, although the terms first, second, etc.may be used herein to describe various elements, these elements shouldnot be limited by these terms. These terms are only used to distinguishone element from another. For example, a first electrode could be termeda second electrode, and, similarly, a second electrode could be termed afirst electrode, without departing from the scope of theimplementations. The first electrode and the second electrode are bothelectrodes, but they are not the same electrode.

Reference throughout this specification to “one example,” “an example,”“exemplary example,” or the like in the singular or plural means thatone or more particular features, structures, or characteristicsdescribed in connection with an example is included in at least oneexample of the present disclosure. Thus, the appearances of the phrases“in one example” or “in an example,” “in an exemplary example,” or thelike in the singular or plural in various places throughout thisspecification are not necessarily all referring to the same example.Furthermore, the particular features, structures, or characteristics inone or more examples may include combined in any suitable manner.

The description of the present application has been presented forpurposes of illustration and description, and is not intended to beexhaustive or limited to the invention in the form disclosed. Manymodifications, variations, and alternative implementations will beapparent to those of ordinary skill in the art having the benefit of theteachings presented in the foregoing descriptions and the associateddrawings. The embodiment was chosen and described in order to bestexplain the principles of the invention, the practical application, andto enable others skilled in the art to understand the invention forvarious implementations and to best utilize the underlying principlesand various implementations with various modifications as are suited tothe particular use contemplated. Therefore, it is to be understood thatthe scope of claims is not to be limited to the specific examples of theimplementations disclosed and that modifications and otherimplementations are intended to be included within the scope of theappended claims.

What is claimed is:
 1. A method of decoding a video signal, comprising:receiving the video signal that includes a first component and a secondcomponent in a first picture frame; receiving, from the video signal, aplurality of sample offsets associated with the second component in thefirst picture frame; deriving a first class index for the secondcomponent from a first set of one or more samples of the first componentrelative to each sample of the second component; selecting a firstsample offset from the plurality of sample offsets for the secondcomponent according to the first class index; and obtaining across-component offsetted sample value of the second component based onthe first sample offset in the first picture frame.
 2. The method ofclaim 1, further comprising: deriving a second class index for thesecond component in a second region in the first picture frame from asecond set of one or more samples of the first component relative toeach sample of the second component; selecting a second sample offsetfrom the plurality of sample offsets for the second component accordingto the second class index; and obtaining a cross-component offsettedsample value of the second component based on the second sample offsetin the second region in the first picture frame; wherein deriving thefirst class index for the second component in the first picture frame isin a first region in the first picture frame.
 3. The method of claim 2,wherein deriving a first class index for the second component from afirst set of one or more samples of the first component relative to eachsample of the second component comprises: deriving the first class indexbased on a first set of one or more classifiers for the first set of oneor more samples of the first component, and deriving a second classindex for the second component in a second region in the first pictureframe from a second set of one or more samples of the first componentrelative to each sample of the second component comprises: deriving thesecond class index based on a second set of one or more classifiers forthe second set of one or more samples of the first component.
 4. Themethod of claim 1, wherein the first picture frame is divided into aplurality of regions, and a different class index is used for each ofthe plurality of regions.
 5. The method of claim 4, further comprising:receiving a syntax element to indicate which class index is used foreach of the plurality of regions in the first picture frame, wherein thesyntax element is switched at a CTU level.
 6. The method of claim 1,wherein the first component is a luma component, and the secondcomponent is a chroma component.
 7. The method of claim 1, furthercomprising: receiving the video signal that includes the first componentand the second component in a second picture frame; and obtaining across-component offsetted sample value of the second component in thesecond picture frame based on an offset set index indicating whichpreviously decoded offset set in the first picture frame stored inmemory is used for the second picture frame.
 8. The method of claim 1,further comprising: receiving, the video signal that includes the firstcomponent and the second component in a second picture frame; receiving,from the video signal, a plurality of sample offsets associated with thesecond component in the second picture frame; deriving a second classindex for the second component from a second set of one or more samplesof the first component relative to each sample of the second component;selecting a second sample offset from the plurality of sample offsetsfor the second component according to the second class index; andobtaining a cross-component offsetted sample value of the secondcomponent based on the second sample offset in the second picture frame;wherein the first set of one or more samples is defined differently fromthe second set of one or more samples.
 9. The method of claim 3, whereinone of the first set of one or more classifiers is derived from firstparameters for selecting a first band corresponding to the first set ofone or more samples of the first component from a first number ofdivided bands of a dynamic range of sample values of the firstcomponent, and one of the second set of one or more classifiers isderived from second parameters for selecting a second band correspondingto the second set of one or more samples of the first component from asecond number of divided bands of the dynamic range of the sample valuesof the first component.
 10. The method of claim 9, wherein selecting afirst sample offset from the plurality of sample offsets for the secondcomponent according to the first class index comprises: selecting thefirst sample offset from the plurality of sample offsets for the secondcomponent corresponding to the first selected band; and selecting asecond sample offset from the plurality of sample offsets for the secondcomponent according to the second class index comprises: selecting thesecond sample offset from the plurality of sample offsets for the secondcomponent corresponding to the second selected band.
 11. The method ofclaim 9, wherein each of the first parameters is switched at one or moreof SPS, APS, PPS, PH, SH, CTU, and CU level, and each of the secondparameters is switched at one or more of SPS, APS, PPS, PH, SH, CTU, andCU level.
 12. The method of claim 11, wherein the first parameterscomprise a position of the first set of one or more samples and thefirst number, and the second parameters comprise a position of thesecond set of one or more samples and the second number.
 13. The methodof claim 8, further comprising: receiving, the video signal thatincludes the first component and the second component in a third pictureframe; and obtaining a cross-component offsetted sample value of thesecond component in the third picture frame based on an offset set indexindicating which previously decoded offset set in other picture framesis used for the third picture frame, wherein a first offset set isstored in memory identified by a first offset set index and a secondoffset set is stored in the memory identified by a second offset setindex for next use.
 14. The method of claim 8, further comprising:receiving, the video signal that includes the first component and thesecond component in a third picture frame; and obtaining across-component offsetted sample value of the second component in thethird picture frame based on an offset set index indicating whichpreviously decoded offset set in other picture frames is used for thethird picture frame, wherein the second component comprises a Cbcomponent and a Cr component, and offset set indexes used for the Cbcomponent and the Cr component are different in the third picture frame.15. The method of claim 1, wherein the first set of one or more samplesof the first component relative to each sample of the second componentis a collocated sample of the first component relative to each sample ofthe second component.
 16. The method of claim 1, wherein the first setof one or more samples of the first component relative to each sample ofthe second component is a neighboring sample of a collocated sample ofthe first component relative to each sample of the second component. 17.The method of claim 1, wherein receiving the video signal comprisesreceiving a syntax element that indicates whether Cross-component SampleAdaptive Offset (CCSAO) is enabled for the video signal.
 18. Anelectronic apparatus comprising: one or more processing units; memorycoupled to the one or more processing units; and a plurality of programsstored in the memory that, when executed by the one or more processingunits, cause the electronic apparatus to perform operations comprising:receiving, the video signal that includes a first component and a secondcomponent in a first picture frame; receiving, from the video signal, aplurality of sample offsets associated with the second component in thefirst picture frame; deriving a first class index for the secondcomponent from a first set of one or more samples of the first componentrelative to each sample of the second component; selecting a firstsample offset from the plurality of sample offsets for the secondcomponent according to the first class index; and obtaining across-component offsetted sample value of the second component based onthe first sample offset in the first picture frame.
 19. The apparatus ofclaim 18, wherein the operations further comprising: deriving a secondclass index for the second component in a second region in the firstpicture frame from a second set of one or more samples of the firstcomponent relative to each sample of the second component; selecting asecond sample offset from the plurality of sample offsets for the secondcomponent according to the second class index; and obtaining across-component offsetted sample value of the second component based onthe second sample offset in the second region in the first pictureframe; wherein deriving the first class index for the second componentin the first picture frame is in a first region in the first pictureframe.
 20. A non-transitory computer readable storage medium storing aplurality of programs for execution by an electronic apparatus havingone or more processing units, wherein the plurality of programs, whenexecuted by the one or more processing units, cause the electronicapparatus to perform operations comprising: receiving, the video signalthat includes a first component and a second component in a firstpicture frame; receiving, from the video signal, a plurality of sampleoffsets associated with the second component in the first picture frame;deriving a first class index for the second component from a first setof one or more samples of the first component relative to each sample ofthe second component; selecting a first sample offset from the pluralityof sample offsets for the second component according to the first classindex; and obtaining a cross-component offsetted sample value of thesecond component based on the first sample offset in the first pictureframe.